1. Field of the Invention
This invention relates to leather coating compositions, particularly those compositions for dyeing, staining, or otherwise modifying the color of leather.
2. Background of the Prior Art
In recent years numerous advancements have been made in processes for coloring leather. U.S. Pat. No. 5,354,341 disclosed a mixture of at least 2 metal-free dyes having a trichromatic formulation that enhances the brilliance of the finished color on the leather. The dyes carry a single sulfo or carboxyl group and molecular weight of 450 to 700. The dyeing time routinely requires 30 to 180 minutes to dry.
U.S. Pat. No. 5,376,147 likewise disclosed improvements in the trichromatic technique by employing dyes having two sulfo or carboxyl groups and molecular weights between 790 to 1100, but the dyeing process still required 30 to 180 minutes to dry.
U.S. Pat. No. 6,290,866 disclosed that an aqueous coating composition comprising copolymerized acetoacetate or acetoacetamide monomer and copolymerized carboxylic acid monomer, in contact with metal oxide, hydroxide, or carbonate provided aesthetically pleasing and protective dried coatings on leather that had good embossability and wet-flex resistance. No enhancements in processing time at ambient temperature were noted; however, when drying at elevated temperatures of about 200° F. (93.3° C.), drying time still required at least 2.0 minutes.
U.S. Pat. No. 6,387,291 discloses raising the glass transition temperature of the above-described aqueous coating compositions by increasing the amount of copolymerized acetoacetate or acetoacetamide. Again the leather products required 2 minutes or 120 seconds to dry at elevated temperatures of 93.3° C.
U.S. Pat. No. 6,471,885 discloses a multistage emulsion polymer for an aqueous leather coating composition, said polymer containing copolymerized monoethylenically-unsaturated nonionic monomer which are predominantly acrylic, i.e. monomers selected from, e.g. esters of (meth) acrylic acid. It also contains copolymerized monoethylenically-unsaturated carboxylic acid monomer. Again, however, the coated leather required 120 seconds to dry at 90° C.
Currently, state-of-the art aqueous coating compositions employed for dyeing leather employ dye mixtures of all types dispersed in propylene glycol monomethyl ether. Even with predominantly acrylic polymers, certain vehicles have been used for enhancing dye penetration, for example polytetrafluoroethylene, (i.e. teflon), and carboxylic acid derivatives such as dibutyl phthalate have been employed as an elastomeric plasticizer to help soften the leather. However, the enhanced dye penetration still requires, at ambient temperature, leather dyeing process times of 5 to 10 minutes and higher before dry surface is obtained. Also, the quality of the finish is not as soft and natural as desired, even though dibutyl phthalate is used.
An improved leather dyeing process with enhanced drying or “flash time” while still providing aesthetically pleasing soft natural coatings with good wet-flex resistance is a long felt need in the industry.